Charge coupled devices (CCDs) are image capture devices that generally offer superior characteristics such as small size, light weight and low power consumption in comparison to other conventional image capture devices. Accordingly, CCDs are commonly used in broadcasting or domestic video cameras, monitoring cameras, and digital still cameras.
As manufacturing and designing techniques have progressed, the density of CCDs has generally increased, resulting in reduced size. As the size of CCDs has reduced, however, the levels of the signals produced by the image capture elements in CCDs have generally become smaller. Therefore, it has become desirable to use output buffers with high gain to produce signals usable for video processing and other purposes from such weak signals.
A typical output buffer is illustrated in FIG. 1. The output buffer includes an input source follower circuit including respective driving and load NMOS transistors M1, M2 which are biased between a power supply voltage VDD and a signal ground. An input signal, e.g., a signal produced by a horizontal transfer section of a CCD image capture device, is applied to the gate terminal of the driving transistor M1, while a control signal Vg is applied to the gate terminal of the load transistor M2. A voltage produced on the source terminal of the driving transistor M1 is generated responsive to the input signal Vin, and is applied to a second stage source follower circuit including driving and load transistors M3 and M4. The second stage source follower circuit similarly drives a third stage source follower circuit including driving and load transistors M5, M6, producing an output signal Vout. Achieving high gain from such a circuit can be problematic.